iBooks update indicator?

Attached is an image of my iBooks library on my iPad (with the titles covered). At the bottom right on the screen, the digit 1 appears in a red circle. The "1 in red circle" does not appear on any individual book. Does anyone know what this is, and how to make it go away?

I thought it might indicate that an upgrade is available for one of my books. But I don't know how to upgrade iBooks titles on my iPad. I do know how to upgrade iBooks titles on my Mac, and I have done this. I use the same iCloud account on my Mac and my iPad, so I expected the little red circle on my iPad to disappear, but it's still there. I also re-synced my iPad to my Mac via iTunes, but the red circle still appears.

I know this is a trivial issue, but if anyone knows how to get rid of the red circle, pls let me know. Thx